1. Field of the Invention
This invention relates to an apparatus and a method for treating (etching, cleaning, etc.) the surfaces of substrates by using vapor of a surface treating liquid mixture having two or more components, and particularly it relates to a surface treating apparatus and a method using vapor wherein the composition of vapor to be used can be easily controlled.
2. Description of the Related Arts
A surface treating apparatus using vapor which is of interest to the present invention is disclosed in Japanese Patent Laying-Open No. 61-148820.
According to this bulletin, a surface treating liquid mixture of composed of hydrogen fluoride, water and alcohol is stored in a storage tank installed under a treating chamber, and in use, the liquid mixture is heated for evaporation. The vapor mixture is fed to substrates to treat their surfaces.
What is most important in surface treatment of substrates is to maintain the composition of the vapor mixture throughout one batch cycle. One reason is that a variation in the composition will cause a variation in the etching rate. As a result, uniform surface treatment of substrates becomes impossible. Another reason is that when a number of substrates are treated, yield varies.
However, the aforesaid bulletin discloses nothing about a method for controlling the composition of vapor mixture to maintain it at a fixed value.
A method for controlling the composition of vapor mixture is disclosed in U.S. Pat. No. 4,749,440.
According to this bulletin, an anhydrous hydrogen fluoride source, a water source and a dry nitrogen gas source are separately prepared. Water from the water source is metered by a metering diaphragm pump and fed to a water vapor chamber. The dry nitrogen gas from the dry nitrogen gas source has its flow rated controlled by a first flow controller and fed to the water vapor chamber. The water vapor resulting from the water being heated to a predetermined temperature for evaporation in the water vapor chamber is mixed with the dry nitrogen gas to provide a water vapor-containing nitrogen gas. This water vapor-containing nitrogen gas is mixed with dry nitrogen gas from the dry nitrogen gas source having its flow rate controlled by a second flow controller and the mixture is fed to an aspirator.
On the other hand, anhydrous hydrogen fluoride gas (a reactive gas for surface treatment) resulting from anhydrous hydrogen fluoride being heated at a predetermined temperature in the anhydrous hydrogen fluoride source is fed to the aspirator through a third flow controller. In the third flow controller, controlled heating is effected for preventing liquefaction or condensation of anhydrous hydrogen fluoride gas and maintained the anhydrous state.
After the water vapor-containing nitrogen gas, dry nitrogen gas and anhydrous hydrogen fluoride gas which have been fed to the aspirator have been mixed together, they are fed to substrates in a reaction chamber (or treating chamber).
In the system described above, in order to control the composition of the vapor mixture, various control are performed: water flow control by the metering diaphragm pump, temperature control in the water vapor chamber, flow control of dry nitrogen gas by the first and second flow controllers, temperature control in the anhydrous hydrogen fluoride source, and flow control and temperature control by the third flow controller.
However, since the composition is controlled by separate control of the amount of each component, the arrangement of the entire control system is very complicated. As a result, it is not easy to provide a vapor mixture of required composition, and to maintain the composition of the vapor mixture at a required value.
Since the composition of the vapor mixture is controlled by feeding hydrogen fluoride gas from the hydrogen fluoride gas source and water vapor from the water source, the concentration of hydrogen fluoride gas in the hydrogen fluoride gas source is very high.
In fact, according to said U.S. Patent, industrial hydrogen fluoride having a concentration of 99,99% is used. If it is desired to use a commercially available solution composed of 49% hydrogen fluoride and 51% water instead of said industrial hydrogen fluoride, it is necessary to heat it to provide dry hydrogen fluoride before it can be used.
However, the use of anhydrous hydrogen fluoride of such high concentration would entail serious danger affecting people's lives if it should leak.